This invention relates to ion trap mass spectrometer.
Mass spectrometers are used to determine the identity and quantity of components that make up a solid, gas, or aqueous sample. A mass spectrometer typically uses the ratio of the ion mass (m) to its charge (z) for analyzing and separating ions. The ion mass is typically expressed in atomic mass units or Daltons (Da) and the ion charge (z) represents the number of electric charges of the ion. One type of mass spectrometer is the quadrupole ion trap mass spectrometer (QITMS), which can be used for analyzing the mass of atomic, molecular and cluster ions. A QITMS typically has a ring electrode and two end-cap electrodes. In operation, a fixed radio-frequency (e.g., approximately 1 MHz) voltage is applied between the ring electrode and the end-cap electrodes to create a time-varying electromagnetic field to confine the ions within a confinement region.
Another type of mass spectrometer is the aerosol time-of-flight mass spectrometer (ATOFMS), which can be used to determine the size and chemical compositions of single aerosol particles in real time. An ATOFMS uses two laser beams to measure the velocity of individual particles in an aerosol beam. The particle""s aerodynamic size is determined from the measured velocity. The chemical composition of each particle is then analyzed using a laser desorption/ionization time-of-flight mass spectrometer. The ATOFMS provides a mass spectrum of the chemical compositions of the particles being analyzed, but reveals little information about the mass/charge ratio of the particles themselves.
In general, in one aspect, the invention features an ion trap that includes two end-cap electrodes and a ring electrode. The ring electrode is positioned between the two end-cap electrodes to form a confinement region. An ion (or a charged particle) from an ion source is confined within the confinement region when an audio frequency voltage is applied between the ring electrode and the end-cap electrodes. The ion is ejected from the ion trap when the amplitude of the audio frequency voltage is increased.
Implementations of the invention may include one or more of the following features. One of the end-cap electrode includes an ion entrance aperture to allow the charged particle to enter into the confinement region, and the other end-cap electrode includes an ion ejection aperture to allow the charged particle to exit the confinement region. The ring electrode includes an observation aperture to allow observation of the movement of the charged particle within the confinement region. The ion trap is used with a light detection module to detect light scattered from the charged particle after it is ejected from the ion trap. The ion source is positioned above the ion trap and includes a needle, a capillary, and a differential pumping region. The needle is aligned along a vertical axis above the capillary, and the capillary is aligned along the vertical axis above the differential pumping region. The capillary and the differential pumping region are connected to electric ground, and the needle is connected to a DC voltage. The audio frequency voltage is in a frequency range between about 50 and 2000 hertz. The ion has a mass in the range between about 1 mega-dalton and 10,000 mega-daltons. The audio frequency voltage has an amplitude in the range between about 400 and 1700 volts.
In general, in another aspect, the invention features a method that includes introducing a charged particle into an ion trap having two end-cap electrodes and a ring electrode positioned between the end-cap electrodes. An audio frequency voltage having a first amplitude is applied between the ring electrode and the end-cap electrodes to generate an electromagnetic field that confines the charged particle within a confinement region. The amplitude of the audio frequency voltage is increased to a second amplitude to eject the charged particle from the ion trap.
Implementations of the invention may include one or more of the following features. A secular frequency of the motion of the charged particle inside the confinement region is measured. A mass-to-charge ratio of the charged particle is calculated based on the second amplitude of the audio frequency voltage and the measured secular frequency.
In general, in another aspect, the invention features a mass spectrometer that includes an ion source, and an ion trap, and a light detection module. The ion trap has two end-cap electrodes and a ring electrode. The ring electrode is positioned relative to the end-cap electrodes to confine a charged particle from the ion source within a confinement region when an audio frequency voltage having a first amplitude is applied between the ring electrode and the two end-cap electrodes. The charged particle is ejected from the ion trap when the audio frequency voltage increases to a second amplitude. The ejected charged particle is detected by the light detection module.
In general, in another aspect, the invention features a method that includes introducing a charged particle into an ion trap of a mass spectrometer, the ion trap having two end-cap electrodes and a ring electrode positioned between the two end-cap electrodes. An audio frequency voltage having a frequency of f and an amplitude of Vac is applied between the ring electrode and the end-cap electrodes to generate an electromagnetic field that confines the charged particle within a confinement region. A secular frequency xcfx89 representing the oscillation frequency of the motion of the charged particle within the confinement region is measured. The amplitude of the audio frequency voltage is then increased to a second amplitude Veject to eject the charged particle from the ion trap. A calibration parameter qeject is calculated based on Vac, Veject,f, and xcfx89. The calibration parameter is used to calculate a mass-to-charge ratio of other charged particles that are introduced into the ion trap and ejected from the ion trap.
Implementations of the invention may include one or more of the following features. The calibration parameter qeject is calculated using the equation             q      eject        =                            V          eject                          V                      a            ⁢                          xe2x80x83                        ⁢            c                              ⁢                        4          ⁢                                    2              ⁢                              xe2x80x83                                              ⁢          ω                Ω              ,
where xcexa9=2xcfx80f.
The radius r of the ring electrode is measured, and a mass-to-charge ratio of another charged particle is calculated using the equation             m      /      z        =                  4        ⁢                  V          eject2                                      q          eject                ⁢                  r          2                ⁢                  Ω          2                      ,
where Veject2 is the amplitude of the audio frequency voltage when the other charged particle is ejected from the ion trap.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.